Our objectives in this renewal application are to bring the insights gained over the past eight years of studying the donor-specific transfusion (DST) mechanisms to bear on the clinical problem of kidney allograft loss due to chronic rejection > 2 yr post-transplant. These insights include: a) the prevention of sensitization via development of anti-idiotypic network response to DST: b) the prevention of early (< 6 mo post-transplant) acute rejection coincident with the development of donor-specific hyporesponsiveness of cytotoxic T lymphocytes; c) susceptibility of HLA mismatched transplants to late graft loss > 2 years post-transplant due to chronic rejection; and d) the resistance of liver transplants to the development of late chronic rejection that affects other organ allografts. The project is divided into four parts: 1) we will utilize purified HLA class I and purified anti-HLA class I antibodies to analyze the role of T-helper and B cell subsets (IgM and IgG) in the development of anti-class I and anti-idiotypic responses; 2) we will analyze the mechanism of donor-specific CTL hyporesponsiveness by comparing CD4+ T-helper and CD8+/cytotoxic-suppressor cell function in bulk culture and in limiting dilution analysis of PBL; 3) we will compare the production of Ab-I (anti-HLA class I), Ab-2, and corresponding TH and B cell responses between liver and kidney transplant recipients, paying particular attention to the correlation with soluble class I levels in the serum; and 4) to determine if a continuous source of soluble HLA class I can inhibit primed T-helper and B cell responses, we will reconstitute scid mice with PBL from highly sensitized patients, and subject them to treatment with recombinant soluble HLA class I to determine the effect on human skin allograft rejection and anti-HLA IgG response. The information generated by all four sections of this project will be used to develop new strategies for graft prolongation using donor-specific antigen manipulation of the host.